Classification may be defined as the separation of solid particles into two or more products according to their velocities when falling through a fluid medium, such as water. The velocity of the particles depends on their size, shape and density. Particles with a higher density and larger size will settle down quickly compared to particles with a lower density and smaller size.
Hydraulic classification is the separation of particles in a tank by specific gravity, utilizing the action of rising water currents to separate the particles into a light fraction and into a heavier fraction. Heavier and larger particles settle down and leave the vessel through an underflow outlet while lighter and finer particles leave the vessel with most of the water as the water overflows from an upper end of the vessel. Hydraulic classifiers are widely used in sand and glass industry.
A problem with hydraulic classification is that turbulence is generated in the tank as the slurry to be separated is introduced into the tank, typically under gravity. Such turbulence disturbs the flow pattern of particles within the tank and reduces the efficiency of the separation process.